Zhuding Qiu

Danghe area (western Gansu, China) biostratigraphy and implications for depositional history and tectonics of northern Tibetan Plateau

The Danghe area in western Gansu Province is at the focal point of interaction of the northeastern end of the left-lateral Altyn Tagh Fault and growing ramp thrusts of the Danghe Nanshan Mountains along the northern rim of the Qinghai-Tibetan Plateau. With a thick sequence of Tertiary sediments and associated fossil records, the Danghe area is one of few places on the Tibetan Plateau that can offer integrated studies of its tectonic history, depositional environment, and biological records, including vertebrate and plant fossils. Past studies, however, have not been able to capitalize on the paleontological data or were often misguided by an outdated notion of regional chronology. Incorrect age estimates in these studies often have profound effects on tectonic interpretations. We present new stratigraphic and paleontologic evidence from the Danghe area and demonstrate a new chronologic scheme that indicates a much younger age for the majority of the sediments. We recognize three packages of sediments in the Danghe area: (1) Oligocene Paoniuquan (new name) Formation, basal, predominantly fine-grained, dark purple mudstones and siltstones that include the classic Yindirte Fauna, which forms the basis of the Tabenbulukian mammal age; (2) early Miocene to early late Miocene Tiejianggou (new name) Formation that contains a new platybelodont proboscidean in a coarsening-upward sequence terminating in a massive conglomerate; and (3) an unnamed late Neogene formation that contains another coarsening-upward sequence. In light of our new stratigraphic framework, we reinterpret a previously published magnetic column for the middle sequence in Xishuigou as representing chrons C6n through C4Ar corresponding to a span of about 20-9.3 Ma, much younger than has been realized so far. This new chronological framework suggests a depositional history of the Danghe Nanshan that spans at least the early Oligocene through late Miocene, to possibly Pliocene. Early depositions during the Oligocene through middle Miocene are dominated by fine-grained sediments indicating a distant source of sedimentation. Paleontological data suggest a relatively dry environment, as is typical of northern China today. By late Miocene (around 9-12 Ma), the Danghe area began to receive coarse sediments and was much closer to the mountain front. The new stratigraphic framework indicates an earlier onset of sedimentation than has generally been assumed, but also suggests the presence of sediments much younger than many have realized

Magnetostratigraphy and paleoenvironmental record of Tertiary deposits of Lanzhou Basin

The Tertiary deposits of the Yongdeng section, the Lanzhou Basin were studied by means of magnetostratigraphy. The magnetostratigraphic sequences from Paleocene to Miocene were established, and the time scale for the corresponding stratigraphy and mammalian faunas was also established by referring to their correlation to the GPTS of BKSA95 (The geomagnetic polarity time scale which was thoroughly revised and updated by Berggreen et al. in 1995). In the end the paleo-environmental records and their relationship with the uplifting of the Tibetan Plateau were discussed. The results are as follows. The top boundary of the Xiliugou Formation is dated to be 51 Ma, i.e. Eocene. The Yehucheng Formation covers the time span of 51—31.5 Ma, i.e. from late Early Eocene to early Early Oligocene. The Xianshuihe Formation covers the time span of 31.5—15 Ma, i.e. from Early Oligocene to Middle Miocene. The lower member of the Xianshuihe Formation spans from 31.5 to 20.0 Ma, i.e. from Early Oligocene to early Early Miocene. The middle member is from 20.0 to 16.5 Ma, i.e. in the middle Early Miocene.